Treatment of aqueous suspension of vinyl chloride polymers

ABSTRACT

Treatment of aqueous suspensions of vinyl chloride polymers containing vinyl chloride monomer so as to inhibit wet-foaming therein, by adding a glyceride of an optionally substituted saturated or unsaturated carboxylic acid containing 6 to 20 carbon atoms (e.g. glycerol monoricinoleate). Solid-foaming can also be inhibited by adding a wetting agent (such as partially hydrolysed polyvinyl acetate PVA) and a preferred formulation for effecting the treatment comprises an aqueous emulsion of the glyceride having PVA as the emulsion stabilizer.

The present invention relates to a method of treating an aqueoussuspension of a vinyl chloride polymer to inhibit foaming, and to anaqueous emulsion formulation for use in said treatment.

Vinyl chloride polymers, by which are meant vinyl chloride homopolymersand copolymers, may be produced by suspension polymerisation atmoderately elevated temperatures, optionally under reflux (using director externally mounted condensers) to provide additional coolingcapacity. The suspension product is subject to a number of operations toremove residual vinyl chloride, since, in view of the toxic nature ofthis substance, it is now recognized that vinyl chloride polymers shouldcontain as little residual vinyl chloride monomer as is possible.Residual vinyl chloride may be largely removed by conventional venting,and optional pressure reduction, following by stripping (e.g. usingsteam) the suspension (not necessarily in the polymerisation reactor) toremove most of the residual vinyl chloride.

It has been found that such polymerisation under reflux and in any caseeach of the above-mentioned operations for removing vinyl chloride oftenresults in the formation of `wet-foam` and `solid foam`. The wet-foam iswhat is conventionally considered as ordinary foam, namely anagglomeration of finely divided gas bubbles dispersed in a liquid (inthis case water) and containing no or very little solid material. Thedry-foam is a floating `cream` of vinyl chloride polymer which comprisesdense polymer particles made buoyant as a result of attached vinylchloride gas bubbles. This cream, or `solid-foam`, is liable to becarried over into the condenser (if used) or into the lines of theventing/pressure reduction system or stripping equipment causing blockedcondensers or pipelines, damaged equipment (such as vacuum pumps),unacceptable losses in polymer and contamination problems when gradesare changed; the solid-foam also inhibits the most effective possibleremoval of residual vinyl chloride from the polymer with the equipmentthat is available. The solid-foaming problem during stripping isparticularly serious and is exacerbated when stripping is carried outunder reduced pressure.

Conventional anti-foam agents, while effective for inhibiting thewet-foaming, are substantially ineffective in reducing thesolid-foaming.

It is disclosed in Research Disclosure 15779 (May 1977) that thissolid-foaming problem may be overcome by adding a wetting agent forvinyl chloride polymers such as partially hydrolysed polyvinyl acetate(hereinafter termed PVA) to the suspension before the commencement of atleast one of the aforementioned processes which cause solid-foaming,rather than by the uneconomic and often largely ineffective control ofoperational conditions (such as slow venting, using only slight pressurereduction, or using a very slow throughput of steam when stripping).

However, it is further disclosed in Research Disclosure 15779, that theuse of PVA as described while preventing solid-foaming, can promote theformation of ordinary form, i.e. wet-foam containing no or very littlesolid material.

While conventional commercially available anti-foams may be used toinhibit this wet-foaming (and indeed it is disclosed in ResearchDisclosure 15779 to add such anti-foams during operations such asstripping to inhibit wet-foaming), their presence as residues in theproduct polymer often adversely affects one or more properties of thepolymer so produced, e.g. heat stability, volume resistivity and powderflow. Moreover such conventional anti-foams are often difficult todisperse in water and are sometimes difficult to handle.

In contrast, it has been disclosed in Research Disclosure 19516 (July1980) that plasticisers such as diisoctyl phthalate are effectiveanti-wet-foams when stripping vinyl chloride polymers. It is furtherdisclosed in Research Disclosure 19516 that the plasticiser'seffectiveness may be improved by using an aqueous dispersion of theplasticiser which includes a fatty acid ester, preferably a lower alkyl(C1 to C5) ester of a C8-20 saturated fatty acid such as ethylpalmitate. Unfortunately, however, we have found that such dispersionstend not to be storage stable.

I have now discovered a method for overcoming the problems ofwet-foaming of an aqueous suspension of a vinyl chloride polymer, evenif such wet-foaming is actively promoted by also treating the aqueoussuspension for the purpose of inhibiting solid foaming, which methoddoes not incur the disadvantageous effects resulting from the use of theconventional anti-foams and which can be effected using a storage-stableadditive formulation.

According to a first aspect of the present invention there is provided amethod of treating an aqueous suspension of a vinyl chloride polymer,produced by suspension polymerisation and containing vinyl chloridemonomer, to inhibit wet-foaming therein, which method comprises addingto the suspension a glyceride of an optionally substituted saturated orunsaturated carboxylic acid containing 6 to 20 carbon atoms. (For thesake of clarity, it is here stated that it is the carboxylic acid thatcontains 6 to 20 carbon atoms).

The utility of glycerides as anti-foams to prevent wet-foaming issurprising; normally such materials are used as lubricants for vinylchloride polymers. In one group of such glycerides each is a glycerideof an unsaturated carboxylic acid containing 6 to 20 carbon atoms. Inanother group each glyceride is a glyceride of an unsaturated orsaturated carboxylic acid containing 14 to 20 carbon atoms. In a furthergroup each is a monoglyceride of a saturated or unsaturated carboxylicacid containing 6 to 20 carbon atoms. The glyceride may be substitutedor unsubstituted in the carboxylic acid component; if substituted thesubstituent is preferably at least one hydroxy or alkoxy group. Theglyceride is preferably a monocarboxylate. Preferred examples ofglycerides to employ are monocarboxylates derived from unsubstituted orhydroxy-substituted oleic acids; preferred examples of these areglycerol monoricinoleate and glycerol monooleate.

The glyceride may be added before, during or after the commencement ofsuspension polymerisation or at least one of the operations to releaseresidual vinyl chloride described hereinbefore i.e. it may be added toprevent the formation of wet-foam and/or to remove it after itsformation (and the term "inhibiting" is intended to embrace both theprevention of foaming and the destruction of foaming after itsformation).

The glyceride does not adversely affect the polymer properties (e.g.heat stability, volume resistivity or powder flow) of the vinyl chloridepolymer so formed. The glycerides are exceptionally convenient tohandle. For example they may be added as a solution in an organicsolvent such as methanol. More usually, however, they are added as anoil-in-water emulsion; such an emulsion may readily be obtained bywater-dilution of a concentrate emulsion (probably a water-in-oilemulsion) of the water-immiscible glyceride which is stabilised with awater-soluble PVA or cellulosic compound; the concentrate may typicallycontain 10 to 60%, more preferably 25 to 55%, by weight (based on thetotal weight of emulsion) of glyceride and 3 to 20% by weight (based onthe weight of glyceride) of water-soluble PVA or cellulosic compound.The concentrate is stable under a wide range of storage conditions,particularly where the organic phase remains liquid down to 0° C. sothat it is unlikely to freeze and break the emulsion, and may be storedwithout any separation; it may be formed e.g. by employing a high shearmixer. The concentrate may be diluted with water to a very stableoil-in-water emulsion (which will not e.g. coagulate in the mix tank oraddition lines) that is stabilised by the PVA or cellulosic derivativefrom the concentrate and contains e.g. 2 to 8% by weight of theglyceride (based on the total weight of emulsion, and 3 to 20% by weightof water-soluble PVA compound (based on the weight of glyceride) for useaccording to the invention.

The amount of glyceride to add in the method of the invention is notcritical and the optimum quantity will depend on the scale employed. Atypical range to us is 30 to 2000 parts per million (ppm) based on theweight of vinyl chloride polymer in the slurry, particularly 50 to 700ppm.

In a second aspect of the present invention, the aqueous suspension inwhich wet-foaming is inhibited is one to which an anti-dry-foam, that isa wetting agent for vinyl chloride polymers, particularly awater-soluble PVA or cellulosic, is also added.

Thus, according to a second aspect of the present invention there isprovided a method of treating an aqueous suspension of a vinyl chloridepolymer, produced by suspension polymerisation and containing vinylchloride monomer, to inhibit wet-foaming and dry-foaming therein, whichmethod comprises adding to the suspension a wetting agent for vinylchloride polymer and a glyceride of an optionally substituted saturatedor unsaturated carboxylic acid containing 6 to 20 carbon atoms. Thewetting agent for vinyl chloride may in particular be a water-solublePVA or cellulosic compound (and if such a material is also used as thesuspension stabiliser for the vinyl chloride polymerisation, it is to beunderstood that the material added to inhibit dry-foaming is additionalto that used as the suspension stabiliser present from the start ofpolymerisation).

Suitable quantities of glyceride in this aspect of the invention are thesame as those in the first aspect.

The wetting agent for vinyl chloride polymer, particularly awater-soluble PVA or cellulosic compound, is used to preventsolid-foaming substantially in the same manner as the use of PVA forthis purpose described in Research Disclosure 15779.

Where the anti-wet-foam of the first aspect of the present invention isin the form of an emulsion stabilised with a water-soluble PVA orcellulosic compound and obtainable from a concentrate emulsion (asdescribed hereinbefore), the PVA or cellulosic compound stabilising theemulsion may itself be also employed as a wetting agent for inhibitingsolid-foaming. Accordingly in such an embodiment, the PVA- orcellulosic-stabilised emulsion of the glyceride inhibits bothsolid-foaming and wet-foaming--although of course the emulsion must beadded at an appropriate stage in the solid-foam-forming operation, vizbefore or just after a solid form starts to form so that solid-foamingis substantially inhibited. Alternatively such an emulsion may augmentor be additional to wetting agent added to prevent solid-foaming, inwhich case it need not necessarily be added before or just aftersolid-foam formation.

Accordingly, there is further provided according to the invention aconcentrate aqueous emulsion formulation for use, after dilution, ininhibiting wet- and dry-foaming in an aqueous suspension of a vinylchloride polymer produced by aqueous suspension polymerisation andcontaining vinyl chloride monomer, which concentrate aqueous emulsionformulation comprises a glyceride of an optionally substitutedcarboxylic acid for inhibiting wet-foaming and a water-soluble PVA orcellulosic derivative serving as the emulsion stabiliser and also forinhibiting dry-foaming.

There is yet further provided according to the invention an aqueousemulsion formulation for use, without dilution, in inhibiting wet- anddry-foaming in an aqueous suspension of a vinyl chloride polymerproduced by aqueous suspension and containing vinyl chloride monomer,which aqueous emulsion formulation comprises a glyceride of anoptionally substituted carboxylic acid for inhibiting wet-foaming and awater-soluble PVA or cellulosic derivative serving as the emulsionstabiliser and also for inhibiting dry-foaming.

The amount of wetting agent to employ in the method of the invention isnot critical and the optimum quantity will depend upon the scaleemployed. A typical range to use is 30-400 ppm based on the weight ofvinyl chloride polymer in the slurry, preferably 50-250 ppm. The wettingagent, if not also used as the stabiliser for an emulsion containingglyceride is conveniently injected as a dilute aqueous solution.Mixtures of various wetting agents may also be employed.

A water-soluble PVA if used in the method of the invention may have anysuitable degree of hydrolysis and the optimum molar degree of hydrolysisfor the particular polymerisation system being employed should bedetermined by experiment. For example, the molar degree of hydrolysiscould be from 65 to 90%, often from 70 to 85% and particularly about 72to 73%. Typical cellulosic compounds if used are methyl cellulose,carboxymethyl cellulose and hydroxyethyl cellulose.

The polymerisation process for the production of vinyl chloride polymerin aqueous suspension may be conventional and any suitable free-radicalmonomer-soluble initiator or a combination of initiators may be used inthe method of the invention. Suitable initiators include acyl peroxidessuch as lauroyl peroxide and acetyl cyclohexyl sulphonyl peroxide, azocompounds such as azodiisobutyronitrile and2,2'-azo-bis-2,4-dimethylvaleronitrile, and dialkyl peroxidicarbonatessuch as diethyl peroxydicarbonate, diisopropyl peroxydicarbonate,di-t.butyl cyclohexyl peroxydicarbonate and dicetyl peroxydicarbonate.Any suitable suspension agent may be used; typical ones includeprotective colloids such as cellulose derivatives and partiallyhydrolysed polyvinyl acetates.

The present invention is illustrated by the following examples. Unlessotherwise specified all parts and percentages are by weight.

EXAMPLE 1

An unstripped slurry of a granular PVC homopolymer was preparedconventionally (polymerisation temperature 58° C.; non refluxconditions).

To about 2 liters of the cold slurry was added 8 ml of a 4% aqueoussolution of PVA of molar degree of hydrolysis 72% and the slurrytransferred to a small steam stripping vessel suitable for laboratoryscale evaluations. This vessel consisted of a glass cylinder (4.5 literin volume) equipped with a stirrer at its base (to prevent the PVCsuspension settling) and having three entry ports near its base: one fora thermometer, the second to inject stream into the slurry, and thethird to drain slurry from the vessel after the termination ofstripping. With the stirrer operating the slurry was charged to thestripping vessel via a charge funnel (closable to atmosphere) at the topof the cylinder. The slurry was heated up by injecting steam from asteam generator into the slurry. No solid-foaming was observed (incomparative runs without the addition of the PVA, the evolution of vinylchloride gas caused solid foaming between 50° and 80° C.) butwet-foaming occurred when the slurry temperature rose to above 50° C.;this was destroyed by the injection of a methanolic solution of 1000parts per million (ppm) of glycerol monoricinoleate through the chargefunnel, the additive precipitating as fine particles so as to beuniformly dispersed in the slurry. The vinyl chloride gas evolved wasvented to a fume cupboard via a condenser, the monomer take-off linealso branching to a separating funnel for the collection of an solidscarry over. Heating was continued until the slurry started to boil at100° C.

The resulting polymer (after drying) was found to have a volumeresistivity (VR) of 114 ohm cm (as measured according to BS 2782, Method202A) as compared to a value of 113 ohm cm for the same polymer strippedwithout the antifoam addition. The thermal stability of the polymer(using various tests) was also virtually the same as that of controlpolymer stripped without the antifoam. Using another sample of vinylchloride polymer (taken from a different batch) and 600 ppm of injectedglycerol monoricinoleate, a VR of 176 ohm cm was achieved as compared toa VR of 160 ohm cm for the same polymer stripped without the addition ofthe antifoam.

By contrast, the use of several commercially available antifoams inplace of the glycerol monoricinoleate (injected as bought from themanufacturer) caused a significant and unacceptable lowering of VR inthe resulting polymer. Also some commercially available antifoams werefound to be poorly dispersible in water leading to a less effectiveavoidance of wet-foam.

EXAMPLE 2

An aqueous-based concentrate emulsion formulation of glycerolmonoricinoleate was prepared by adding 80 g of the material to 120 g ofa 10% aqueous solution of PVA (molar degree of hydrolysis 72%) withvigorous stirring using a high shear mixer. The concentrate emulsionformuation was thus a mixture of 40% by weight glycerol monoricinoleate(based on the total weight of emulsion) and 60% by weight of the aqueousPVA solution (based on the total weight of emulsion and corresponding to15% by weight of PVA based on the weight of glyceride), and had theappearance of a thick white cream (probably from its appearance a waterin oil emulsion). The concentrate formulation was stored in jars andshowed no sign of instability after 3 months in the laboratory. Samplesof antifoam for evaluation could be readily prepared simply by dilutingsamples of the concentrate with water using mild stirring; for thefollowing experiment 1 part of concentrate was diluted with 7 parts ofwater to give an oil-in water emulsion formulation of the glycerolmonoricinoleate containing 5% by weight of glyceride (based on the totalweight of emulsion). The diluted concentrate was quite stable and couldbe left for several days without stirring.

A sample of the antifoam (diluted concentrate prepared as describedabove) was evaluated using the stripping technique described in Example1 with a slurry of a vinyl chloride homopolymer (polymerisationtemperature 66° C.; non reflux conditions) and with an aqueous solutionof PVA again being added to the cold slurry before stripping to inhibitdry-foaming. 500 ppm of glycerol monoricinoleate (and correspondingly 75ppm of PVA) (based on vinyl chloride polymer) contained in the addeddiluted concentrate were used. Effective prevention of dry-foaming anddestruction of wet-foaming was achieved without any adverse effect onvolume resistivity and thermal stability.

EXAMPLE 3

The antifoam prepared as described in Example 2 (diluted concentrate),but on a larger scale, was evaluated when stripping slurries obtainedfrom a variety of plant-scale vinyl chloride homo- and copolymerisationsprepared using various polymerisation temperatures (55° C. to 72° C.;non-reflux conditions) and PVA (molar degree of hydrolysis 72%) assuspension agent. Various levels of antifoam were employed (varying from30 to 600 ppm of glycerol monoricinoleate based on vinyl chloridepolymer), and the formation of wet-foam was always destroyed orprevented by the addition of an appropriate amount of antifoam. Solidfoaming was also prevented (aqueous PVA solution again having beeninjected before stripping) and there was no adverse effect on volumeresistivity, thermal stability, and powder flow.

EXAMPLE 4

This was again a laboratory scale experiment, this time to simulate thetroublesome effect in vinyl chloride polymerisations carried out with areflux condenser wherein there is often an unwelcome surge ofsolid-foaming around the time of pressure drop which rises up theautoclave into the condenser, causing the blockage of the latter. Theequipment used was a cylindrical glass vessel (approx. volume 3 liters)surrounded by a cooling/heating water jacket and fitted at the top witha condenser, a top entry stirrer, and a sealable injection port, and atthe bottom with a sintered glass base (No. 2 porosity) above a bottomopening connected by tubing to a cylinder of vinyl chloride. The methodwas to simulate the often excessive solid-foaming at pressure drop in acondenser-fitted reactor by adding unstripped PVC slurry to the vesselwith the stirrer running, heating up the slurry using the water jacket,and when solid-foaming started to form and rise up the vessel, to bubblein vinyl chloride through the sinter (creating small bubbles of the gas)to further increase the solid-foaming.

This technique was used on 1 liter of an unstripped slurry of a granularPVC homopolymer (polymerisation temperature 57° C.) charged to thevessel. When the temperature reached about 50° C. vinyl chloride wasbubbled in to cause severe dry-foaming which rose up and started toenter the condenser. The antifoam prepared as described in Example 2(diluted concentrate) was then injected, a total of 300 ppm of glycerolmonoricinoleate and correspondingly 45 ppm of PVA (based on dry polymerweight) being used. Not only was wet-foaming prevented but the severedry-foaming was entirely destroyed.

Similar runs using a solution containing only PVA or other types ofwetting agent, while preventing solid-foaming, did not prevent theformation of wet-foaming which in some cases was very severe.

I claim:
 1. Method of treating an aqueous suspension of a vinyl chloridepolymer, produced by aqueous suspension polymerisation and containingresidual vinyl chloride monomer, to inhibit wet-foaming .Iadd.anddry-foaming .Iaddend.therein, which method comprises adding to saidaqueous suspension of a vinyl chloride polymer containing residual vinylchloride monomer, a glyceride of a saturated or unsaturated carboxylicacid, containing 6 to 20 carbon atoms .Iadd.to inhibit wet-foaming and awetting agent to inhibit dry-foaming, and wherein said glyceride isadded to the suspension in the form of an oil-in-water emulsion andwherein at least part of the wetting agent used to inhibit dry-foamingis a water-soluble PVA or cellulosic compound which additionally servesas an emulsion stabilizer for the oil-in-water emulsion of the glyceridethat is added to the suspension.Iaddend.. .[.2. Method of treating anaqueous suspension of a vinyl chloride polymer, produced by aqueoussuspension polymerization and containing residual vinyl chloridemonomer, to inhibit wet-foaming and dry-foaming therein, which methodcomprises adding to the aqueous suspension of polymer containingresidual vinyl chloride monomer, a wetting agent for vinyl chloridepolymer and a glyceride of a saturated or unsaturated carboxylic acidcontaining 6 to 20 carbon atoms..].
 3. Method according to .[.either.].claim 1 .[.or claim 2.]. wherein the carboxylic acid component of theglyceride is unsaturated.
 4. Method according to claim 1 wherein theglyceride used is a glycerol monocarboxylate.
 5. Method according toclaim 1 wherein the carboxylic acid component of the glyceride used isunsubstituted or hydroxy substituted.
 6. Method according to claim 1wherein the glyceride used is glycerol monoricinoleate or glycerolmonooleate. .[.7. Method according to claim 1 wherein the glyceride isadded to the suspension in the form of a solution in an organic solventor in the form of an oil-in-water emulsion..].
 8. Method according toclaim 1 wherein 50 to 2000 parts per million, based on the weight ofvinyl chloride polymer, of glyceride is added to the suspension. .[.9.Method according to claim 2 wherein the wetting agent used is awater-soluble PVA or cellulosic compound..].
 10. Method according toclaim .[.2.]. .Iadd.1 .Iaddend.wherein the PVA has a molar degree ofhydrolysis of 65 to 90%. .[.11. Method according to claim 2 wherein atleast part of the wetting agent used is a water-soluble PVA orcellulosic compound which additionally serves as an emulsion stabiliserfor an oil-in-water emulsion of the glyceride that is added to thesuspension..].
 12. Method according to claim .[.2.]. .Iadd.1.Iaddend.wherein 30 to 400 parts per million, based on the weight ofvinyl chloride polymer, of wetting agent is added to the suspension.